Organic semiconductors: a theoretical characterization of the basic parameters governing charge transport

Carbon-Carbon Linked Organic Frameworks: An Explicit Summary and Analysis

Organic frameworks with carbon-carbon (CC) linkage are an important class of materials owing to their outstanding chemical stability and extended π-electron delocalization resulting in unique optoelectronic properties. In the first part of this review article, the design principles for the bottom-up synthesis of 2D and 3D sp/sp² CC linked organic frameworks are summarized. Representative reaction methodologies, such as Knoevenagel condensation, Aldol condensation, Horner-Wadsworth-Emmons reaction, Wittig reaction, and coupling reactions (Ullmann, Suzuki, Heck, Yamamoto, etc.) are included. This is discussed in the context of their reaction mechanism, reaction dynamics, and whether and why resulting in an amorphous or crystalline product. This is followed by a discussion of different state-of-the art bottom-up synthesis methodologies, like solvothermal, interfacial, and solid-state synthesis. In the second part, the structure-property relationships in CC linked organic frameworks with representative examples of organocatalysis, photo(electro)catalysis, energy storage and conversion, magnetism, and molecular storage and separation are analyzed. The importance of linkage type, building blocks, topology, and crystallinity of the framework material in connection with the structure-property relationship is highlighted. Finally, brief concluding remarks are presented based on the key development of bottom-up synthetic methods and provide perspectives for future development in this field.

High throughput processing of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) organic semiconductors

The deposition of organic semiconductors (OSCs) using solution shearing deposition techniques is highly appealing for device implementation. However, when using high deposition speeds, it is necessary to use very concentrated OSC solutions. The OSCs based on the family of dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) have been shown to be excellent OSCs due to their high mobility and stability. However, their limited solubility hinders the processing of these materials at high speed. Here, we report the conditions to process alkylated DNTT and the S-shaped π-core derivative S-DNTT by bar-assisted meniscus shearing (BAMS) at high speed (i.e., 10 mm s^-1). In all the cases, homogeneous thin films were successfully prepared, although we found that the gain in solubility achieved with the S-DNTT derivative strongly facilitated solution processing, achieving a field-effect mobility of 2.1 cm² V^-1 s^-1, which is two orders of magnitude higher than the mobility found for the less soluble linear derivatives.

Charge transport in liquid crystal network of terthiophene-siloxane block molecules

In their thermotropic liquid-crystalline state, molecular semiconductors can show charge transport with high carrier mobility. Polymerization of the corresponding mesogens into a cross-linked network often deteriorates the charge transport. Here, we report that mesogens consisting of a terthiophene core and discrete oligodimethylsiloxane side-chains terminated by acrylate units can be photopolymerized in the columnar phase with retention of nanoscale order and charge transport capabilities. We argue that the strong tendency for microphase segregation protects the semiconducting block from reacting with free radicals during polymerization. This work provides new insights into the design of electroactive materials with charge transport properties.

Photochromic Semiconductors: Bottom-Up Strategy to Construct Type II-Stacking Viologen π-Aggregates

Semiconductor conductivities depend largely on the crystal structures and the associated electronic structures. If the electronic structures can be switched reversibly in the same crystal structure, then a drastic conductivity change may be controllable. The effect of electron transfer (ET) on semiconductor conductivity remained elusive so far. In this work, a series of two pillared inorganicorganic hybrid photochromic semiconductors (PSCs), [(CQ)Pb₃X₆(H₂O)]·2H₂O [X = Cl (1) and Br (2), CQ = N-4,4'-bipyridiniopropionate (viologen)], with II-stacking viologen π-aggregates, are constructed by a bottom-up self-assembly strategy through inorganic skeleton-directed intercalation and intermolecular noncovalent interaction. The conductivities are abnormally "invariant" after photoinduced ET, breaking the convention that the generation of radicals favors conductivity. The abnormally "invariant" conductivities are mainly derived from approximate electronic couplings before and after ET between II-stacking viologen π-aggregates.

Generalization on Entropy-Ruled Charge and Energy Transport for Organic Solids and Biomolecular Aggregates

Herein, a generalized version of the entropy-ruled charge and energy transport mechanism for organic solids and biomolecular aggregates is presented. The effects of thermal disorder and electric field on electronic transport in molecular solids have been quantified by entropy, which eventually varies with respect to the typical disorder (static or dynamic). Based on our previous differential entropy (h s )-driven charge transport method, we explore the nonsteady carrier energy flux principle for soft matter systems from small organic solids to macrobiomolecular aggregates. Through this principle, the synergic nature of charge and energy transport in different organic systems is addressed. In this work, entropy is the key parameter to classify whether the carrier dynamics is in a nonsteady or steady state. Besides that, we also propose the formulation for unifying the hopping and band transport, which provides the relaxation time-hopping rate relation and the relaxation time-effective mass ratio. The calculated disorder drift time (or entropy-weighted carrier drift time) for hole transport in an alkyl-substituted triphenylamine (TPA) molecular device is 9.3 × 10-7 s, which illustrates nuclear dynamics-coupled charge transfer kinetics. The existence of nonequilibrium transport is anticipated while the carrier dynamics is in the nonsteady state, which is further examined from the rate of traversing potential in octupolar molecules. Our entropy-ruled Einstein model connects the adiabatic band and nonadiabatic hopping transport mechanisms. The logarithmic current density at different electric field-assisted site energy differences provides information about the typical transport (whether trap-free diffusion or trap-assisted recombination) in molecular devices, which reflects in the Navamani-Shockley diode equation.

Introducing a new 7-ring fused diindenone-dithieno[3,2-b:2',3'-d]thiophene unit as a promising component for organic semiconductor materials

A novel π-conjugated molecule, EtH-T-DI-DTT is reported, which is fused, rigid, and planar, featuring the electron-rich dithieno[3,2-b:2',3'-d]thiophene (DTT) unit in the core of the structure. Adjacent to the electron-donating DTT core, there are indenone units with electron-withdrawing keto groups. To enable solubility in common organic solvents, the fused system is flanked by ethylhexylthiophene groups. The material is a dark, amorphous solid with an onset of absorption at 638 nm in CH2Cl2 solution, which corresponds to an optical gap of 1.94 eV. In films, the absorption onset wavelength is at 701 nm, which corresponds to 1.77 eV. An ionisation energy of 5.5 eV and an electron affinity of 3.3 eV were estimated by cyclic voltammetry measurements. We have applied this new molecule in organic field effect transistors. The material exhibited a p-type mobility up to 1.33 × 10-4 cm2 V-1 s-1.

Non-Equal Ratio Cocrystal Engineering to Improve Charge Transport Characteristics of Organic Semiconductors: A Case Study on Indolo[2,3-a]carbazole

Rare studies of cocrystal engineering have focused on improving carrier mobility of organic semiconductors mainly because of the generation of ambipolarity, the alteration of the charge carrier polarity or the reduction of electronic couplings. Herein, we utilize indolo[2,3-a]carbazole (IC) as the model compound and 2,6-diphenylanthraquinone (DPAO) and 9-fluorenone (FO) as the coformers to construct IC2-DPAO and IC-FO cocrystals with 2 : 1 or 1 : 1 ratios, respectively, through hydrogen bonds and donor-acceptor interactions. Interestingly, the more appropriate packing structure, possessing not only enhanced electronic couplings but also increased intermolecular distances, is achieved in IC2-DPAO, which shows an improved carrier mobility of 0.11 cm²  V^-1  s^-1 by four orders of magnitude relative to the IC crystal. These results suggest that non-equal ratio cocrystal engineering opens up the possibility to develop organic semiconductors with enhanced charge transport behaviors.

Modulating singlet fission through interchromophoric rotation

Singlet fission (SF) is a spin-allowed, exciton-multiplying phenomenon that can be utilized to improve the efficiency of organic solar cells. It is well-understood that SF is sensitive to the local crystal morphology and an appropriately balanced coupling is essential to facilitate efficient SF. In this study, we show how the interchromophoric rotation selectively modulates the interaction between the monomer frontier molecular orbitals, promoting both fast and exothermal SF. We evaluate the effective electronic coupling for SF (V_SF), the square of which is proportional to the SF rate, and the effective energies of the Frenkel exciton (FE/S₁S₀) and triplet pair exciton (TT) in a terrylene dimer model. Optimal interplanar rotation of the chromophoric moieties in slip-stacked arrangements pulls the effective energy of the TT state below that of the FE state. Consequently, SF is favored over competing pathways such as excimer formation, thereby enhancing the overall triplet yield. This work represents a step towards improvising the molecular design guidelines for SF and understanding the importance of interchromophoric rotation over the conventional slip-stacked arrangements for achieving favorable intermolecular electronic coupling towards efficient SF.

Increase of Fluorescence of Humic-Like Substances in Interaction with Cd(II): a Photoinduced Charge Transfer Approach

Described is the enhancement of fluorescence intensity due to the interaction of a humic-like substance (HLS 1%) extracted from process water (PW) and Cd(II) ions in aqueous solution. Using Canonical Polyadic/Parallel Factor Analysis (CP/PARAFAC), two main components were seen that contributed to fluorescence, the first one increased it and the second one kept it constant in both static and dynamic fluorescence studies. Two-dimensional FTIR analysis indicated that the interaction of HLS 1% and Cd(II) ions occurred in the following order of affinity with the groups: C-O bonds in polysaccharides > C-O bonds in carboxylic acid. The results obtained suggest that the increase in fluorescence intensity and lifetime suggest a photoinduced charge transfer (PCT) between Cd(II) ions and carboxylic acid groups present in HLS 1%.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

Centimeter-scale hole diffusion and its application in organic light-emitting diodes

In conventional organic light-emitting diodes (OLEDs), current balance between electron and hole transport regions is typically achieved by leakage of the major carrier through the devices or by accumulation of the major carrier inside the devices. Both of these are known to reduce performances leading to reduction of efficiency and operation stability due to exciton-polaron annihilation, etc. We found that hole diffusion in a centimeter-scale can be achieved in a PEDOT:PSS layer via composition and interface engineering. This ultralong distance hole diffusion enables substantially enhanced hole diffusion current in the lateral direction perpendicular to the applied electric field in typical organic optoelectronic devices. By introducing this lateral hole diffusion layer (LHDL) at the anode side of OLEDs, reduced carrier accumulation, improved efficiency, and enhanced operation stability are demonstrated. The application of the LHDL provides a third strategy for current balancing with much reduced harmful effects from the previous two approaches.

The Molecular Ordering and Double-Channel Carrier Generation of Nonfullerene Photovoltaics within Multi-Length-Scale Morphology

The success of nonfullerene acceptor (NFA) solar cells lies in their unique physical properties beyond the extended absorption and suitable energy levels. The current study investigates the morphology and photophysical behavior of PBDB-T donor blending with ITIC, 4TIC, and 6TIC acceptors. Single-crystal study shows that the π-π stacking and side-chain interaction dictate molecular assembly, which can be carried to blended films, forming a multi-length-scale morphology. Spontaneous carrier generation is seen in ITIC, 4TIC, and 6TIC neat films and their blended thin films using the PBDB-T donor, providing a new avenue of zero-energy-loss carrier formation. The molecular packing associated with specific contacts and geometry is key in influencing the photophysics, as demonstrated by the charge transfer and carrier lifetime results. The 2D layer of 6TIC facilitates the exciton-to-polaron conversion, and the largest photogenerated polaron yield is obtained. The new mechanism, together with the highly efficient blending region carrier generation, has the prospect of the fundamental advantage for NFA solar cells, from molecular assembly to thin-film morphology.

A Potential Iterative Approach to 1,4-Dihydro-N-Heteroacene Arrays

A new method for the synthesis of substituted 1,4-dihydrophenazines is reported and the structure of N-butyl-5-methyl-3-nitro-5,10-dihydrophenazine is proven by an X-ray single-crystal structure determination.

Efficient Intermolecular Charge Transport in π-Stacked Pyridinium Dimers Using Cucurbit[8]uril Supramolecular Complexes

Intermolecular charge transport through π-conjugated molecules plays an essential role in biochemical redox processes and energy storage applications. In this work, we observe highly efficient intermolecular charge transport upon dimerization of pyridinium molecules in the cavity of a synthetic host (cucurbit[8]uril, CB[8]). Stable, homoternary complexes are formed between pyridinium molecules and CB[8] with high binding affinity, resulting in an offset stacked geometry of two pyridiniums inside the host cavity. The charge transport properties of free and dimerized pyridiniums are characterized using a scanning tunneling microscope-break junction (STM-BJ) technique. Our results show that π-stacked pyridinium dimers exhibit comparable molecular conductance to isolated, single pyridinium molecules, despite a longer transport pathway and a switch from intra- to intermolecular charge transport. Control experiments using a CB[8] homologue (cucurbit[7]uril, CB[7]) show that the synthetic host primarily serves to facilitate dimer formation and plays a minimal role on molecular conductance. Molecular modeling using density functional theory (DFT) reveals that pyridinium molecules are planarized upon dimerization inside the host cavity, which facilitates charge transport. In addition, the π-stacked pyridinium dimers possess large intermolecular LUMO-LUMO couplings, leading to enhanced intermolecular charge transport. Overall, this work demonstrates that supramolecular assembly can be used to control intermolecular charge transport in π-stacked molecules.

Interfacial Charge Transfer Influences Thin-Film Polymorphism

The structure and chemical composition are the key parameters influencing the properties of organic thin films deposited on inorganic substrates. Such films often display structures that substantially differ from the bulk, and the substrate has a relevant influence on their polymorphism. In this work, we illuminate the role of the substrate by studying its influence on para-benzoquinone on two different substrates, Ag(111) and graphene. We employ a combination of first-principles calculations and machine learning to identify the energetically most favorable structures on both substrates and study their electronic properties. Our results indicate that for the first layer, similar structures are favorable for both substrates. For the second layer, we find two significantly different structures. Interestingly, graphene favors the one with less, while Ag favors the one with more electronic coupling. We explain this switch in stability as an effect of the different charge transfer on the two substrates.

Significant Enhancement of the Electrical Conductivity of Conjugated Polymers by Post-Processing Side Chain Removal

The processability and electronic properties of conjugated polymers (CPs) have become increasingly important due to the potential of these materials in redox and solid-state devices for a broad range of applications. To solubilize CPs, side chains are needed, but such side chains reduce the relative fraction of electroactive material in the film, potentially obstructing π-π intermolecular interactions, localizing charge carriers, and compromising desirable optoelectronic properties. To reduce the deleterious effects of side chains, we demonstrate that post-processing side chain removal, exemplified here via ester hydrolysis, significantly increases the electrical conductivity of chemically doped CP films. Beginning with a model system consisting of an ester functionalized ProDOT copolymerized with a dimethylProDOT, we used a variety of methods to assess the changes in polymer film volume and morphology upon hydrolysis and resulting active material densification. Via a combination of electrochemistry, X-ray photoelectron spectroscopy, and charge transport models, we demonstrate that this increase in electrical conductivity is not due to an increase in degree of doping but an increase in charge carrier density and reduction in carrier localization that occurs due to side chain removal. With this improved understanding of side chain hydrolysis, we then apply this method to high-performance ProDOT-alt-EDOT_x copolymers. After hydrolysis, these ProDOT-alt-EDOT_x copolymers yield exceptional electrical conductivities (∼700 S/cm), outperforming all previously reported oligoether-/glycol-based CP systems. Ultimately, this methodology advances the ability to solution process highly electrically conductive CP films.

Engineering of flat bands and Dirac bands in two-dimensional covalent organic frameworks (COFs): relationships among molecular orbital symmetry, lattice symmetry, and electronic-structure characteristics

Two-dimensional covalent organic frameworks (2D-COFs), also referred to as 2D polymer networks, display unusual electronic-structure characteristics, which can significantly enrich and broaden the fields of electronics and spintronics. In this Focus article, our objective is to lay the groundwork for the conceptual description of the fundamental relationships among the COF electronic structures, the symmetries of their 2D lattices, and the frontier molecular orbitals (MOs) of their core and linker components. We focus on monolayers of hexagonal COFs and use tight-binding model analyses to highlight the critical role of the frontier-MO symmetry, in addition to lattice symmetry, in determining the nature of the electronic bands near the Fermi level. We rationalize the intriguing feature that, when the core unit has degenerate highest occupied MOs [or lowest unoccupied MOs], the COF highest valence band [or lowest conduction band] is flat but degenerate with a dispersive band at a high-symmetry point of the Brillouin zone; the consequences of having such band characteristics are briefly described. Multi-layer and bulk 2D COFs are found to maintain the salient features of the monolayer electronic structures albeit with a reduced bandgap due to the interlayer coupling. This Focus article is thus meant to provide an effective framework for the engineering of flat and Dirac bands in 2D polymer networks.

Synergistic effects of side-functionalization and aza-substitution on the charge transport and optical properties of perylene-based organic materials: a DFT study

The physicochemical properties of organic materials are subject to the chemical structure of the molecular unit and the arrangement of molecules in a crystal.

Unraveling Semiconductor Properties of Mixed Stack Donor Acceptor Cocrystals of Pyrene Derivatives and TCNQ: Effect of Crystal Packing versus Super-exchange Mechanism

We have discussed semiconductor property of two reported 2:1 and 1:1 charge transfer donor acceptor cocrystals with mixed ··DDADDA·· stacking. These cocrystals (CCDC 1212856 and 1212858) are comprising of 2-phenyl-3-(pyrene-2-yl)acrylonitrile...

Tuning p-type to n-type Semiconductor Nature by Charge Transfer Cocrystallization: Effect of Transfer Integral vs. Reorganization Energy

In this contribution, 1:2 mixed stack (··DADA·· arrangement) donor acceptor cocrystal comprised of hole transport material CBP (4,4ʹ-bis(9H-carbazole-9-yl)biphenyl) as the donor (D), and TCNQ (7,7ʹ,8,8ʹ-tetracyano-1,4-quinodimethane) as the acceptor (A) was...

The crystalline state of rubrene materials: intermolecular recognition, isomorphism, polymorphism, and periodic bond-chain analysis of morphologies

Crystal structure analysis and lattice energy calculations of 33 rubrenes provide a background for new ideas on synthesis and planning of transport properties.

Theoretical exploration of diverse electron-deficient core and terminal groups in A–DA′D–A type non-fullerene acceptors for organic solar cells

The influences triggered by the structurally diverse electron-withdrawing terminal group and fuse-ring electron-deficient core on the performance of NFAs OSCs are comprehensively investigated by using DFT, TD-DFT and Marcus charge transfer theory.

Folding of aromatic polyamides into a rare intrachain β-sheet type structure and further reinforcement of the secondary structure through host–guest interactions

We report the design, synthesis, and folding of aromatic polyamides into an intrachain β-sheet-like structure. Additionally, the effect of a guest molecule in stabilizing the β-sheet structure has also been demonstrated here.

Impact of Imine Bond Orientations on the Geometric and Electronic Structures of Imine-based Covalent Organic Frameworks

Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized.

Approaching isotropic charge transport of n-type organic semiconductors with bulky substituents

Benzo[de]isoquinolino[1,8-gh]quinolinetetracarboxylic diimide (BQQDI) is an n-type organic semiconductor that has shown unique multi-fold intermolecular hydrogen-bonding interactions, leading to aggregated structures with excellent charge transports and electron mobility properties. However, the strong intermolecular anchoring of BQQDI presents challenges for fine-tuning the molecular assembly and improving the semiconducting properties. Herein, we report the design and synthesis of two BQQDI derivatives with phenyl- and cyclohexyl substituents (Ph-BQQDI and Cy₆-BQQDI), where the two organic semiconductors show distinct molecular assemblies and degrees of intermolecular orbital overlaps. In addition, the difference in their packing motifs leads to strikingly different band structures that give rise to contrasting charge-transport capabilities. More specifically, Cy₆-BQQDI bearing bulky substituents exhibits isotropic intermolecular orbital overlaps resulting in equal averaged transfer integrals in both π-π stacking directions, even when dynamic disorders are taken into account; whereas Ph-BQQDI exhibits anisotropic averaged transfer integrals in these directions. As a result, Cy₆-BQQDI shows excellent device performances in both single-crystalline and polycrystalline thin-film organic field-effect transistors up to 2.3 and 1.0 cm² V^-1 s^-1, respectively.

Pressure-Induced Enhancement of Thermoelectric Performance in Rubrene

In this work, the thermoelectric performance of a typical small-molecule organic semiconductor rubrene under different hydrostatic pressures was studied by first-principles calculation and molecular dynamics simulation. The ZT value of rubrene can reach 1.6 at 400 K due to an unprecedented increase in hole mobility under hydrostatic pressure. The underlying mechanism is ascribed to the suppression of low-frequency phonons (which weakens electron-phonon scattering) and the increase in the intermolecular electronic coupling. The effect of uniaxial stress has also been investigated to confirm this conclusion. Our results provide meaningful insights to understand the relationship between thermoelectric properties and hydrostatic pressure in organic semiconductors.

Mixed Ionic and Electronic Conduction in Small-Molecule Semiconductors

Small-molecule organic semiconductors have displayed remarkable electronic properties with a multitude of π-conjugated structures developed and fine-tuned over recent years to afford highly efficient hole- and electron-transporting materials. Already making a significant impact on organic electronic applications including organic field-effect transistors and solar cells, this class of materials is also now naturally being considered for the emerging field of organic bioelectronics. In efforts aimed at identifying and developing (semi)conducting materials for bioelectronic applications, particular attention has been placed on materials displaying mixed ionic and electronic conduction to interface efficiently with the inherently ionic biological world. Such mixed conductors are conveniently evaluated using an organic electrochemical transistor, which further presents itself as an ideal bioelectronic device for transducing biological signals into electrical signals. Here, we review recent literature relevant for the design of small-molecule mixed ionic and electronic conductors. We assess important classes of p- and n-type small-molecule semiconductors, consider structural modifications relevant for mixed conduction and for specific interactions with ionic species, and discuss the outlook of small-molecule semiconductors in the context of organic bioelectronics.

Pushing the Limits of Flexibility and Stretchability of Solar Cells: A Review

Emerging forms of soft, flexible, and stretchable electronics promise to revolutionize the electronics industries of the future offering radically new products that combine multiple functionalities, including power generation, with arbitrary form factor. For example, skin-like electronics promise to transform the human-machine-interface, but the softness of the skin is incompatible with traditional electronic components. To address this issue, new strategies toward soft and wearable electronic systems are currently being pursued, which also include stretchable photovoltaics as self-powering systems for use in autonomous and stretchable electronics of the future. Here recent developments in the field of stretchable photovoltaics are reviewed and their potential for various emerging applications are examined. Emphasis is placed on the different strategies to induce stretchability including extrinsic and intrinsic approaches. In the former case, engineering and patterning of the materials and devices are key elements while intrinsically stretchable systems rely on mechanically compliant materials such as elastomers and organic conjugated polymers. The result is a review article that provides a comprehensive summary of the progress to date in the field of stretchable solar cells from the nanoscale to macroscopic functional devices. The article is concluded by discussing the emerging trends and future developments.

Exploring the charge injection aptitude in pyrazol and oxazole derivatives by the first-principles approach

Azole derived products acquired significant consideration in everyday life based on their improved biological potential to the semiconducting substances. The research focused in-depth within pyrazol, and oxazole compounds 1–4 concerning charge transport, structural, optical as well as electronic properties. The density functional theory (DFT) along with time-dependent DFT were used for the optimization of their ground state geometries and excitation energies. We also investigated the molecule’s electron coupling constants (|V RP|) as well as electron injection (ΔG inject) values. For better understanding, charge transport and electronic characteristics were performed through quantum chemical computations. The |V RP| and ΔG inject values of pyrazole, as well as oxazole molecules, exhibited that these compounds could be competent for dye-sensitized solar cell applications. The pyrazole higher diagonal band gap enlightening these might have enhanced fill factor (FF) along with short-circuit current density (J sc ). We have also explored the electron injection, energy level offset, dissociation of excitons, and band alignment of studied compounds to shed light on the functionality of these compounds for photovoltaic and semiconductor device applications.

Maximizing the Carrier Mobilities of Metal-Organic Frameworks Comprising Stacked Pentacene Units

Charge transport properties of metal-organic frameworks (MOFs) are of distinct interest for (opto)electronic applications. In contrast to the situation in molecular crystals, MOFs allow an extrinsic control of the relative arrangement of π-conjugated entities through the framework architecture. This suggests that MOFs should enable materials with particularly high through-space charge carrier mobilities. Such materials, however, do not yet exist, despite the synthesis of MOFs with, for example, seemingly ideally packed stacks of pentacene-bearing linkers. Their rather low mobilities have been attributed to dynamic disorder effects. Using dispersion-corrected density functional theory calculations, we show that this is only part of the problem and that targeted network design involving comparably easy-to-implement structural modifications have the potential to massively boost charge transport. For the pentacene stacks, this is related to the a priori counterintuitive observation that the electronic coupling between neighboring units can be strongly increased by increasing the stacking distance.

A general charge transport picture for organic semiconductors with nonlocal electron-phonon couplings

The nonlocal electron-phonon couplings in organic semiconductors responsible for the fluctuation of intermolecular transfer integrals has been the center of interest recently. Several irreconcilable scenarios coexist for the description of the nonlocal electron-phonon coupling, such as phonon-assisted transport, transient localization, and band-like transport. Through a nearly exact numerical study for the carrier mobility of the Holstein-Peierls model using the matrix product states approach, we locate the phonon-assisted transport, transient localization and band-like regimes as a function of the transfer integral (V) and the nonlocal electron-phonon couplings (ΔV), and their distinct transport behaviors are analyzed by carrier mobility, mean free path, optical conductivity and one-particle spectral function. We also identify an "intermediate regime" where none of the established pictures applies, and the generally perceived hopping regime is found to be at a very limited end in the proposed regime paradigm.

Understanding the origin of serrated stacking motifs in planar two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) have attracted significant attention due to their chemical versatility combined with a significant number of potential applications. Of particular interest are two-dimensional COFs, where the organic building units are linked by covalent bonds within a plane. Most properties of these COFs are determined by the relative arrangement of neighboring layers. These are typically found to be laterally displaced, which, for example, reduces the electronic coupling between the layers. In the present contribution we use dispersion-corrected density-functional theory to elucidate the origin of that displacement, showing that the common notion that the displacement is a consequence of electrostatic repulsions of polar building blocks can be misleading. For the representative case of COF-1 we find that electrostatic and van der Waals interactions would, actually, favor a cofacial arrangement of the layers and that Pauli repulsion is the crucial factor causing the serrated AA-stacking. A more in-depth analysis of the electrostatic contribution reveals that the "classical" Coulomb repulsion between the boroxine building blocks of COF-1 suggested by chemical intuition does exist, but is overcompensated by attractive effects due to charge-penetration in the phenylene units. The situation becomes more involved, when additionally allowing the interlayer distance to relax for each displacement, as then the different distance-dependences of the various types of interactions come into play. The overall behavior calculated for COF-1 is recovered for several additional COFs with differently sized π-systems and topologies, implying that the presented results are of more general relevance.

Mobility anisotropy in the herringbone structure of asymmetric Ph-BTBT-10 in solution sheared thin film transistors

Thin films of the organic semiconductor Ph-BTBT-10 and blends of this material with polystyrene have been deposited by a solution shearing technique at low (1 mm s^-1) and high (10 mm s^-1) coating velocities and implemented in organic field-effect transistors. Combined X-ray diffraction and electrical characterisation studies prove that the films coated at low speed are significantly anisotropic. The highest mobility is found along the coating direction, which corresponds to the crystallographic a-axis. In contrast, at high coating speed the films are crystallographically less ordered but with better thin film homogeneity and exhibit isotropic electrical characteristics. Best mobilities are found in films prepared at high coating speeds with the blended semiconductor. This work demonstrates the interplay between the crystal packing and thin film morphology and uniformity and their impact on the device performance.

Interfacial versus Bulk Properties of Hole-Transporting Materials for Perovskite Solar Cells: Isomeric Triphenylamine-Based Enamines versus Spiro-OMeTAD

Here, we report on three new triphenylamine-based enamines synthesized by condensation of an appropriate primary amine with 2,2-diphenylacetaldehyde and characterized by experimental techniques and density functional theory (DFT) computations. Experimental results allow highlighting attractive properties including solid-state ionization potential in the range of 5.33-5.69 eV in solid-state and hole mobilities exceeding 10^-3 cm²/V·s, which are higher than those in spiro-OMeTAD at the same electric fields. DFT-based analysis points to the presence of several conformers close in energy at room temperature. The newly synthesized hole-transporting materials (HTMs) were used in perovskite solar cells and exhibited performances comparable to that of spiro-OMeTAD. The device containing one newly synthesized hole-transporting enamine was characterized by a power conversion efficiency of 18.4%. Our analysis indicates that the perovskite-HTM interface dominates the properties of perovskite solar cells. PL measurements indicate smaller efficiency for perovskite-to-new HTM hole transfer as compared to spiro-OMeTAD. Nevertheless, the comparable power conversion efficiencies and simple synthesis of the new compounds make them attractive candidates for utilization in perovskite solar cells.

Mutually exclusive hole and electron transfer coupling in cross stacked acenes

The topology of frontier molecular orbitals (FMOs) induces highly sensitive charge transfer coupling with variation in the intermolecular arrangement. A consistent optoelectronic property correlated to a specific aggregate architecture independent of the nature of the monomer is a rare phenomenon. Our theoretical investigation on stacked dimeric systems of linear [n]acenes (n = 2-5) and selected non-linear acenes with a D2h point group reveals that the Greek cross (+) stacked orientation, irrespective of the molecular candidate, exhibits mutually exclusive hole and electron transfer couplings. The deactivation of either hole or electron transfer coupling is a consequence of the zero inter-orbital overlap between the highest occupied molecular orbitals (HOMOs) or lowest unoccupied molecular orbitals (LUMOs) of the monomers possessing gerade symmetry. In the Greek cross (+) stacked alignment, the (4n + 2) π-electronic acene systems with an odd number of benzenoids exhibit exclusive electron transfer coupling, while the even numbered acenes exhibit selective hole transfer coupling. The trend is reversed for representative 4n π-electronic acene systems. The effect of mutually exclusive charge transfer coupling in the hopping regime of charge transport was evaluated using semiclassical Marcus theory, and selective charge carrier mobility was exhibited by the Greek cross (+) stacks of the considered acene candidates. Additionally, the characteristic charge transfer coupling of the orthogonal acene stacks resulted in negligible short-range exciton coupling, inciting null exciton splitting at short interplanar distances. Engineering chromophores in precise angular orientations ensuring characteristic emergent properties can have tremendous potential in the rational design of advanced optoelectronic materials.

Effect of Molecular Substitution and Isomerization on Charge-Transport Parameters in Molecular Organic Semiconductors

Charge transport in an organic semiconductor is strongly dependent on the molecular packing motif, which could be modified by the molecular substitutions and molecular isomerization. We constructed a series of benzodithiophene-based organic semiconductor molecules with different silyethyne substitutions and isomers. The existence of different conformations of these molecules is supported by a low isomerization energy barrier from density functional theory. By using Marcus semiclassical theory calculation, we make a comprehensive assessment for the effect of molecular substitution and isomerization on charge transport. We found that the hole mobility of cis-isomer molecular packing can be enhanced by increasing the length of silylethyne substitutions. We demonstrated that a favorable charge-transport material would possess an identical direction of induced ring currents, stable induced magnetic fields, and dominant π-π stacking interaction in their molecular packing motif to ensure good π-overlap area. Our findings will provide direct guidance for developing organic semiconductor materials.

Improving the Robustness of Organic Semiconductors through Hydrogen Bonding

Molecular organization plays an essential role in organic semiconductors since it determines the extent of intermolecular interactions that govern the charge transport present in all electronic applications. The benefits of hydrogen bond-directed self-assembly on charge transport properties are demonstrated by comparing two analogous pyrrole-based, fused heptacyclic molecules. The rationally designed synthesis of these materials allows for inducing or preventing hydrogen bonding. Strategically located hydrogen bond donor and acceptor sites control the solid-state arrangement, favoring the supramolecular expansion of the π-conjugated surface and the subsequent π-stacking as proved by X-ray diffraction and computational calculations. The consistency observed for the performance of organic field-effect transistors and the morphology of the organic thin films corroborate that higher stability and thermal robustness are achieved in the hydrogen-bonded material.